1. Field of the Invention
The present invention relates to a ripple voltage suppression method, and more particularly to a ripple voltage suppression method for DC/DC converter and apparatus thereof.
2. Description of the Related Art
Owing to binding the Kyoto Protocol for reducing greenhouse gas emissions and the increase cost of fossil fuel, renewable energies have the features of low pollution and low greenhouse gas emission, and it has become more and more important. Wind energy, solar energy and fuel cell are the important energy sources of renewable energies.
Now regarding to FIG. 1, a schematic structure illustrated a power conversion system connected between a DC input voltage source and an AC power distribution system or an AC load is shown. Generally, the DC input voltage source is a renewable power source: such as wind power generator, solar cell array or fuel cell system, wherein an electrical power generated by a wind power generator has to be initially converted into a DC power to match the voltage level of DC voltage of DC/AC inverter of the power conversion system. The reason for use of the power conversion system is illustrated as the following. The value of a DC input voltage generated by the wind power or solar power is variable and dependent on the conditions of climate and environment such as wind speed, temperature or illumination of sun, while that generated by fuel cells is also easily to vary due to the changes in load and flow amount of hydrogen and oxygen. Hence, the voltage level of DC input voltage generated by any one of the above-mentioned renewable energies is generally unstable, and the value thereof usually varies largely. Besides, the value of a DC input voltage outputted by the said renewable power source with small capacity is still too low for utilization or transmission. Consequently, a DC input voltage generated by the DC input voltage source has to be converted into an efficient and regular AC power through the power conversion system before being supplied to the AC power distribution system.
Referring to FIG. 1 again, the power conversion system includes a DC/DC converter 91, a DC/AC inverter 92 and a DC capacitor 93. The DC/DC converter 91, which is preferably selected from a boost converter for increasing the voltage level of the DC input voltage to another voltage level suitable for the DC voltage level of the DC/AC inverter 92, converts a DC power with the DC input voltage generated by the DC input voltage source into a regulated DC voltage. The DC/AC inverter 92 further converts the DC power with higher voltage level generated by the DC/DC converter 91 into an AC power for said AC power distribution system or AC load. The DC capacitor 93 electrically connects between the DC/DC converter 91 and the DC/AC inverter 92 in parallel and acts as an energy buffer. The DC/AC inverter 92 can generate a sinusoidal current in phase with that of the AC power distribution system and is injecting into the AC power distribution system. And thus the AC power transmitted to the AC power distribution system has unity power factor. Besides, the DC/AC inverter 92 can also generate a sinusoidal voltage for supplying the AC load.
Besides, more than the average real power, the AC power generated by the DC/AC inverter 92 also has an AC instantaneous power. In detail, when the AC power is transmitted to the AC power distribution system, the frequency of said AC instantaneous power is twice of a fundamental frequency of the AC power distribution system. And thus, the AC instantaneous power builds a ripple voltage on the DC capacitor 93, with the ripple voltage having a frequency equal to a double of the fundamental frequency of the AC power distribution system. On the other hand, when the AC power is transmitted to the AC load, the frequency of said AC instantaneous power is twice of a voltage frequency of the AC power. Hence, the AC instantaneous power builds another ripple voltage on the DC capacitor 93, with the ripple voltage having a frequency twice of the voltage frequency of the AC power distribution system.
As a result, if the DC/DC converter 91 only completes a control for supplying a regulated DC output voltage, the ripple voltage on the DC capacitor 93 will be transmitted to the DC input voltage generated by the DC input voltage source through the DC/DC converter 91. When the DC input voltage source is selected from a solar cell array or wind power generator, a ripple of the DC input voltage induced by the ripple voltage on the DC capacitor 93 causes a ripple voltage on the DC input voltage and thus unable to be fixed at an optimal operation point (maximum power point). Namely, a power generating efficiency of the DC input voltage source is lowered because of the existence of the ripple voltage. When the ripple voltage is superimposed on the DC input voltage source generated by a fuel cell system, this ripple voltage will raise the peak value of the DC input voltage. Said raised peak value of the DC input voltage results in needs of larger amount of fuel and capacity for a fuel cell system to maintain the same average power generated by the ripple-free fuel cell system, which absolutely increases the cost for building and operating the fuel cell system. Besides, the DC input voltage with the ripple voltage also shortens the life of the stacks of the fuel cell system.
Therefore, in order to suppress the ripple voltage caused by the DC/AC inverter 92, two conventional methods to improve have been proposed, which are illustrated as the following. The first one of them was published on IEEE Telecommunications Energy Conference, 2006, with a title of “Active Filtering of Input Ripple Current to Obtain Efficient and Reliable Power from Fuel Cell Sources”. As shown in FIG. 2, through an active DC filter 94 parallelly connecting with the DC/AC inverter 92 at a DC side thereof, a current with a frequency of twice of the fundamental frequency generated by the active DC filter 94 is fed into the DC capacitor 93. Therefore, the ripple voltage caused by the AC instantaneous power and transmitted to the DC input voltage source through the DC capacitor 93 and DC/DC converter 91 is suppressed. However, a cost for the conventional power conversion system is thus raised for an installation of the active DC filter 94.
The second one of them was published on IEEE Transactions on Power Electronics, vol. 22, no. 4, July 2007, with a title of “Low Frequency Current Ripple Technique with Active Control in a Fuel Cell Power System with Inverter Load”. Said conventional method is applied to a controller of the DC/DC converter 91, with a control method for the controller being a current-mode control instead of a voltage-mode control. Therefore, a response speed of the DC/DC converter 91 is raised, and thus the ripple voltage transmitted to the DC input voltage source is suppressed. Specifically, with a current detector, the current-mode control processed by the controller is executed by detecting an input current of the DC/DC converter 91 as a feedback signal. Hence, the input current of the DC/DC converter 91 is formed into a DC current by the controller, which prevents the ripple voltage generated by the DC/AC inverter 92 from entering the DC input voltage source. Besides, in order to outputting a regulated DC voltage, the DC/DC converter 91 further includes a voltage control circuit outside of the current-mode control for completing a function of voltage regulation. Therefore, the controller of the DC/DC converter 91 not only includes the voltage control circuit as an outer loop controller but also provides a current control circuit as an inner loop controller, so as to achieve the ripple voltage suppression as well as the voltage regulation. However, said double loop controller of the DC/DC converter 91 results in a complex circuit structure and an extra cost for the additional current detector. Accordingly, there is a need for redesigning the conventional ripple voltage suppression method for DC/DC converter and the apparatuses for implementation thereof.